The technical problem that is addressed by the present invention has been known since the earliest application of x-rays as an aid in medical diagnostics and the performance of medical procedures. The problem is easily understood with reference to FIG. 1A and FIG. 1B. X-ray source 10 is roughly a point source that emits a cone of x-rays that project an image 40 of radiopaque object 20 on surface 30. Surface 30 is in some cases essentially planar but is usually distorted as a result of the configuration of the equipment used to make the images. The surface 30 can be of any type made sensitive to x-rays, e.g. a sheet of glass or plastic or a thin paper or plastic film coated with a material that fluoresces when struck by x-rays or coated with a photographic emulsion or an electronic device whose surface has an array of pixels such as a CCD device. As can be seen in the figures, the scale of image 40 on surface 30 depends on the distance of identical objects 20 from the source 10 (FIG. 1A) and/or on the angle of the object with reference to planar surface 30 (FIG. 1B). As a result, the surgeon can not accurately measure distances or the size, shape, and orientation of objects in x-ray images and has to rely on intuition and experience to determine these parameters. The problem is especially serious in the case of surgical procedures that must be carried out using frequent x-ray imagery. In this case accurate work is limited by the ability of the surgeon to know exact values of the above mentioned parameters. In the absence of this information, time consuming trial and error is needed to complete the procedure and the lack of accurate measurements has been determined to be one of the causes of failures of orthopedic procedures.
As mentioned above, this problem was recognized very early in the development of the field of medical radiography. In January 1897, only a little over one year after the ground breaking paper by Roentgen that gave the first scientific explanation of the phenomenon that he called x-rays, a patent application that eventually became U.S. Pat. No. 581,540 was filed in the U.S. Patent Office. The invention comprises a grid of radiopaque wires placed between the object being x-rayed (inside a human body) and the planar surface on which the images are recorded and an “angle plate” which is applied to the body to insure parallelism of the x-rays. The object of the invention being to provide “an improved radiographic apparatus whereby the exact location of an invisible object, not permeable or difficulty permeable by the so-called “Roentgen” or “X” rays, may be accurately ascertained and measurements made by which operations necessary for the removal of such objects are controlled and guided”.
In the intervening years since the publication of U.S. Pat. No. 581, 540 and the present, numerous patents have been granted and scientific articles published that provide different solutions to different aspects of the same problem. A brief review of some of these solutions can be found by reviewing the following patents:                U.S. Pat. No. 1,396,920 describes an indicator comprising radiopaque marks on a plane parallel to the object to be observed and the x-ray sensitive plate. In this way the indicator appears on the x-ray image and the known distances between the marks can be used to determine the correct scale of the distances that appear in the image and thus the size of the object can be accurately determined.        U.S. Pat. No. 5,970,119 describes a scaling device comprising an easily observable radiopaque member having radiolucent gaps spaced a known distance apart. The embodiments of the scaling device can be use externally or incorporated into a catheter to allow the device to be manipulated into a position in the vicinity of the anatomical structure to be measured as close as possible to the plane of the structure while being oriented as closely as possible to perpendicular to the x-ray beam.        U.S. Pat. No. 5,052,035 describes a device comprised of a transparent substrate on which is created a grid of parallel radiopaque lines. The film is placed over the area of the body of the patient of interest and an x-ray image is taken. The grid appears in the x-ray image as an overlay on the anatomical structure. The transparent substrate is adapted so that, by use of a marking instrument, marks can be applied to the body. In this way features that appear in the x-ray image can be accurately located on/in the body of the patient.        U.S. Pat. No. 3,706,883 describes an elongated probe (catheter) that includes at least one radiopaque segment of known length. The probe is introduced into the body and is brought into proximity to the object to be measured. The radiopaque portion of the probe appears on the x-ray image next to the object whose size is unknown. The ratio of the apparent length of the radiopaque portion of the probe to its known length provides the scale factor necessary to determine the length of the other objects that appear in the x-ray image.        U.S. Pat. No. 4,005,527 describes a depth gauge comprised of alternating sections of radiopaque and radiolucent material of known length. The depth gauge can be inserted into a hole or cavity to be observed using x-ray methods. The gauge will be seen on the x-ray image and can be used to provide a scale to measure the depth of the hole and dimensions of other features seen in the image. In one embodiment, the depth gauge is the shaft of a drill and serves to enable the surgeon to know the depth of the hole that he has drilled into a bone.        
This brief survey of the prior art gives an indication of a fact of life that is well known to surgeons, i.e. that the solution to the problems first recognized in the earliest days of medical radiography has not yet been found. Each of the solutions proposed to date, while it might represent an improvement over prior proposals or may give adequate results for certain procedures, has not provided an overall solution.
A surgeon using any of the previous measurement techniques, whether involving using a regular ruler to measure objects directly (not through x-ray) or measuring objects on the image itself will experience the same limitations. Measuring objects directly is often problematic since access is limited to the objects measured and measuring on the image itself, besides requiring a calibration, can only provide measurements on the projection of the object and in the projection plane.
While x-ray images are two dimensional and prior art techniques allow reasonably accurate two dimensional measurements in the plane of the image itself, the surgeon would ideally like to have the ability to make three dimensional measurements and measure the objects at any direction he desires. In particular orthopedic surgeons would like to be able to accurately measure objects not in the image plane and to measure objects, without penetrating them, while retaining the measurement accuracy.
It is therefore a purpose of the present invention to provide a ruler which improves upon and overcomes the limitations of prior art rulers used for measuring distances in radiographic images.
It is another purpose of the present invention to provide a ruler which allows a surgeon to make three dimensional measurements and measure objects in a radiographic image at any direction he desires.
It is another purpose of the present invention to provide a ruler which allows a surgeon to accurately measure objects not in the image plane, while retaining the measurement accuracy.
It is another purpose of the present invention to provide a ruler which allows a surgeon to accurately measure objects without penetrating them, while retaining the measurement accuracy.
Further purposes and advantages of this invention will appear as the description proceeds.